Information is recorded or reproduced in or from an optical disk by narrowing down a laser beam to a very-small spot diameter and making the laser beam irradiate a recording surface of an optical recording medium.
To make the laser beam having a certain spot diameter irradiate a recording surface, it is necessary that a distance between an objective lens for condensing the laser beam and an optical disk is kept within a range in which a shift between a focal position of the objective lens and the recording surface falls within a focal depth of the objective lens.
For this reason, an optical disk drive for reproducing data from an optical disk (or recording and reproducing data in and from a rewritable optical disk) or an exposure device for a master optical disk detects a change of the distance between an objective lens and an optical disk, moves the objective lens in the optical-axis direction of a laser beam in accordance with the detection result, and thereby performs control of the distance (focus control).
As the focus control method, for example, the off-axis method, astigmatism method, and knife-edge method have been used so far. All of these methods detect a change of the distance between an objective lens and an optical disk using the light reflected from a signal-recording surface of an optical disk.
To make the spot diameter still smaller in response to needs of high-density optical disk, a numerical aperture larger than that of an objective lens itself (e.g. numerical aperture larger than 1) has recently been realized by intervening an solid immersion lens (SIL) of a shape obtained by cutting off a part of a spherical lens and having a high refractive index between an objective lens and an optical disk facing the spherical surface of the solid immersion lens and the plane opposite to the spherical surface toward the objective lens and optical disk, respectively. Also, it is possible to realize a large numerical aperture by a solid immersion mirror (SIM).
When a numerical aperture is larger than 1, an intensity of a laser beam to irradiate an optical disk extremely lowers if the distance between an SIL and the optical disk exceeds a near field (a range of almost the wavelength of the laser beam). Therefore, it is necessary to perform focus control so that the distance may be kept constant within the range of the near field.
However, in a very small range such as the near field, even if the distance between the SIL and the optical disk changes, a change of the light reflected from the optical disk is very small. Thus, it is difficult to accurately detect a change of the distance between an objective lens and the optical disk in accordance with the change of the reflected light.
Therefore, when setting an SIL between an objective lens and an optical disk, it is difficult to perform high-accuracy focus control by the conventional focus control method.
The same problem occurs in a beam irradiator such as an exposure device used for manufacturing a master optical disk for obtaining an information-recording medium such as an optical disk.
It is an object of the present invention to provide a distance-change detecting method, a distance-change detector, a focus control method and a focus controller, as well as a totally-reflected-light detecting method which enables to perform high-accuracy focus control by accurately detecting a change of the distance between an SIL or SIM and an optical disk even if an optical means for increasing a numerical aperture such as the SIL or SIM is employed.